Polybenzimidazoles are polymers of high thermal stability and excellent resistance to oxidative or hydrolytic degradation. As taught by the published literature, these polymers may be prepared, for example, by melt polymerizing an aromatic tetraamine and a diphenylester or an anhydride of an aromatic or heterocyclic dicarboxylic acid in a one or two stage process; see, for example H. Vogel and C. S. Marvel, Journal of Polymer Science, Vol. L, pages 511-539 (1961); and U.S. Pat. Nos. Re. 26,065; 3,174,947; 3,509,108; 3,551,389; 3,433,772; and 3,655,632. In particular, U.S. Pat. No. 3,551,389 discloses a two stage process for the production of aromatic polybenzimidazoles, in which the monomers are heated at a temperature above 170.degree. C. in a first stage melt polymerization zone until a foamed prepolymer is formed. The foamed prepolymer is cooled, pulverized, and introduced into a second stage polymerization zone where it is heated again to yield a polybenzimidazole polymer product.
It has also been known to prepare polybenzimidazoles from the free dicarboxylic acids or the methyl esters of such acids rather than the phenyl esters or anhydrides in a melt polymerization process. Polybenzimidazoles produced utilizing certain dicarboxylic compounds as monomers have repeating units of the following formula: ##STR1## wherein R is a tetravalent aromatic nucleus with the nitrogen atoms forming the benzimidazole rings being paired upon adjacent carbon atoms, i.e., ortho carbon atoms, of the aromatic nucleus, and R' is a member of the class consisting of an aromatic ring; an alkylene group (preferably having 4 to 8 carbon atoms); and a heterocyclic ring such as pyridine, pyrazine, furan, quinoline, thiophene, and pyran. Depending on whether the dicarboxylic acid moieties in the dicarboxylic monomer component are the same or different, R' may be the same or randomly different among the repeating units along the polymer chain. Moreover, depending on whether one or more than one tetraamine monomer is utilized in the polymerization, R may also be the same or randomly different along the polymer chain.
The following generalized equation illustrates the condensation reaction which occurs in forming the polybenzimidazoles having the recurring units of the foregoing formula: ##STR2## in which R and R' are as previously defined. Such polybenzimidazoles are produced by the reaction of a mixture of (1) at least one aromatic tetraamine containing two groups of amine substituents, the amine substituents in each group being in an ortho position relative to each other, and (2) a dicarboxylic component as indicated in the foregoing equation and as more completely defined hereinafter.
Aromatic tetraamines which may be used, for example, are those with the following formulas: ##STR3## where X represents --O--, --S--, --SO.sub.2, --C--, or a lower alkylene group, such as --CH.sub.2 --, --(CH.sub.2).sub.2 --, or --C(CH.sub.3).sub.2 --. Among such aromatic tetraamines may be mentioned, for example, 1,2,4,5-tetraaminobenzene, 1,2,5,6-tetraaminonaphthalene; 2,3,6,7-tetraaminonaphthalene; 3,3',4,4'-tetraaminodiphenyl methane; 3,3',4,4'-tetraaminodiphenyl ethane; 3,3',4,4'-tetraaminodiphenyl-2,2-propane; 3,3',4,4'-tetraaminodiphenyl thioether; and 3,3',4,4'-tetraaminodiphenyl sulfone. The preferred aromatic tetraamine is 3,3',4,4'-tetraaminobiphenyl.
The compounds which comprise the dicarboxylic component of this invention are defined by the formula: ##STR4## in which the Y's may be hydrogen, aryl or alkyl with no more than 95% of the Y's being hydrogen or phenyl. The dicarboxylic component may therefore consist of a mixture of a free acid with at least at one diester and/or monoester; a mixture of diester(s) and/or monoester(s); or a single dialkyl ester, monoester or mixed aryl-alkyl or alkyl/alkyl ester but can consist completely of free acid or diphenyl ester. When Y is alkyl, it preferably contains 1 to 5 carbon atoms and is most preferably methyl. When Y is aryl, it may be any monovalent aromatic group obtained by filling with hydrogen all the valences but one of the aromatic groups which may be R or R' as disclosed previously, either unsubstituted or substituted with any inert monovalent radical such as alkyl or alkoxy containing 1 to 5 carbon atoms. Examples of such aryl groups are phenyl, naphthyl, the three possible phenylphenyl radicals and the three possible tolyl radicals. The preferred aryl group is usually phenyl.
The dicarboxylic acids which are suitable in free or esterified form as part of the dicarboxylic component as previously described for use in the production of polybenzimidazoles by the process of the present invention include aromatic dicarboxylic acids; aliphatic dicarboxylic acids (preferably, those having 4 to 8 carbon atoms); and heterocyclic dicarboxylic acids wherein the carboxylic groups are substituents upon carbon atoms in a ring compound such as pyridine, pyrazine, furan, quinoline, thiophene, and pyran.
Dicarboxylic acids which may be utilized in free or esterified form as described are aromatic dicarboxylic acids such as those illustrated below: ##STR5## where X is as defined above. For example, the following diacids can suitably be employed; isophthalic acid; terephthalic acid; 4,4'-biphenydicarboxylic acid; 1,4-naphthalene-dicarboxylic acid; diphenic acid (2,2'-biphenyldicarboxylic acid); phenylindandicarboxylic acid; 1,6-napthalenedicarboxylic acid; 2,6-naphthalenedicarboxylic acid; 4,4'-diphenyletherdicarboxylic acid; 4,4'-diphenylsulfonedicarboxylic acid; 4,4'-diphenylthioetherdicarboxylic acid. Isophthalic acid is the dicarboxylic acid which in free or esterified form is most preferred for use in the process of the present invention.
The dicarboxylic component can be one of the following combinations: (1) at least one free dicarboxylic acid and at least one diphenyl ester of a dicarboxylic acid; (2) at least one free dicarboxylic acid and at least one dialkyl ester of a dicarboxylic acid, and (3) at least one diphenyl ester of a dicarboxylic acid and at least one dialkyl ester of a dicarboxylic; and (4) at least one dialkyl ester of a dicarboxylic acid. The dicarboxylic moieties of the compounds of each combination may be the same or different and the alkyl groups of the alkyl esters of combinations (2), (3) and (4) generally contain 1 to 5 carbon atoms and are most preferably methyl.
The dicarboxylic component can be employed in a ratio of about 1 mole of total dicarboxylic component per mole or aromatic tetraamine. However, the optimal ratio of reactants in a particular polymerization system can be easily determined by one of ordinary skill in the art.
Examples of polybenzimidazoles which may be prepared according to the process as described above include:
poly-2,2'-(m-phenylene)-5,5'-bibenzimidazole; PA0 poly-2,2'-(biphenylene-2"2'")-5,5'-bibenzimidazole; PA0 poly-2,2'-(biphenylene-4"4"')-5,5'-bibenzimidazole; PA0 poly-2,2'-(1",1",3"trimethylindanylene)-3"5"-p-phenylene-5,5'-bibenzimidazo le; PA0 2,2'-(m-phenylene)-5,5'-bibenzimidazole/2,2-(1",1",3"-trimethylindanylene)- 5",3"-(p-phenylene)-5,5'-bibenzimidazole copolymer; PA0 2,2'-(m-phenylene)-5,5-bibenzimidazole-2,2'-biphenylene-2",2"'-5,5'-bibenzi midazole copolymer; PA0 poly-2,2'-(furylene-2",5")-5,5'-bibenzimidazole; PA0 poly-2,2'-(naphthalene-1",6")-5,5'-bibenzimidazole; PA0 poly-2,2'-(naphthalene-2",6")-5,5'-bibenzimidazole; PA0 poly-2,2'-amylene-5,5'-bibenzimidazole; PA0 poly-2,2'-octamethylene-5,5'-bibenzimidazole; PA0 poly-2,2'-(m-phenylene)-diimidazobenzene; PA0 poly-2,2'-cyclohexenyl-5,5'-bibenzimidazole; PA0 poly-2,2'-(m-phenylene)-5,5'-di(benzimidazole)ether; PA0 poly-2,2'-(m-phenylene)-5,5'-di(benzimidazole)sulfide; PA0 poly-2,2'-(m-phenylene)-5,5'-di(benzimidazole)sulfone; PA0 poly-2,2'-(m-phenylene)-5,5'-di(benzimidazole)methane; PA0 poly-2,2"-(m-phenylene)-5,5"-di(benzimidazole)propane-2,2; and PA0 poly-ethylene-1,2-2,2"-(m-phenylene)-5,5"-dibenzimidazole)ethylene-1,2
where the double bonds of the ethylene groups are intact in the final polymer.
Poly-1,2'-(m-phenylene)-5,5'-bibenzimidazole, a preferred polymer, can be prepared by the reaction of 3,3',4,4'-tetraaminobiphenyl with a combination of isophthalic acid with diphenyl isophthalate or with a dialkyl isophthalate such as dimethyl isophthalate; a combination of diphenyl isophthalate and a dialkyl isophthalate such as dimethyl isophthalate; or at least one dialkyl isophthalate such as dimethyl isophthalate, as the sole dicarboxylic component.
Although the sintering of polybenzimidazole (PBI) resins in the manner described herein is not known in the art, the compression molding of PBI resins and the sintering of a mixture of PBI polymer and prepolymer has been effected. Levine (Encycl. Polymer Sci. Technol., 11, 188) reported in 1969 the matched metal compression molding of low molecular weight PBI having an inherent viscosity (IV) of 0.05-0.1 dl/g as a 0.4% weight solution in 97% sulfuric acid. The compression product was reported to have a tensile strength (unfilled) of 17000-25000 psi and a compressive strength (yield) of 54 thousand pounds per square inch.
The process for sintering PBI polymers in which a prepolymer was used as a sintering aid is described in U.S. Pat. No. 3,340,325. As described therein, the prepolymer was prepared by reacting a diphenyl ester of an aromatic dicarboxylic acid and an aromatic tetraamine to a degree short of substantial infusibility. The prepolymer is fusible at temperatures in the range of from about 200.degree. F. to about 500.degree. F. The mixture of PBI prepolymer and PBI polymer having a melting point greater than 1500.degree. F. was introduced into a mold with sufficient heat and pressure applied to cause the prepolymer to become fluid and the mixture was maintained under sufficient heat and pressure to cure the prepolymer.
The previously described prepolymer molding processes have two distinct disadvantages. The prepolymer off-gases significant amounts of phenol and water during cure, necessitating the care in tailoring a cure cycle and leading to either high void contents or limited part thickness. The prepolymer also contains detectable amounts of residual 3,3',4,4'-tetraaminobiphenyl (TAB) monomer. As such, care must be taken when handling this material to insure that no worker contact occurs.
In 1985 Jones et al. reported (International Conference on Composite Materials IV, AIME, Warrendale, Pa., p. 1591) the compression molding of PBI polymer at temperatures of 600.degree. to 800.degree. F., pressures of 2000 psi and final hold times of over one hour. The product molded articles had tensile strengths of 7000 psi. Employing the described process, only one part could be made per mold per cycle, with total cycle times limited to one per normal eight hour shift and part thickness was effectively limited to less than one inch.
Ward .sup.1 2 and Harb, et al..sup.3 reported matched metal die compression molding of PBI at temperatures of up to 875.degree. F., pressures of 5,000 to 10,000 psi and cycle time of 4-8 hours which resulted in molded PBI parts, limited to 1/4 inch in thickness, with tensile strength of up to 21,000 psi. In order to achieve high tensile strength, however, it was necessary to utilize a resin with an IV of 1.1 dl/g. Use of resin with lower molecular weight resulted in correspondingly lower tensile strength of the molded article. FNT .sup.1 Ward, B. C., Fabricating Composites '86, SME Composites Group, Baltimore, Md. (September, 1986), EM86-704. FNT .sup.2 Ward, B. C., 32nd International SAMPE Symposium, Anaheim, CA, (Apr. 6-9, 1987) pp. 853-867. FNT .sup.3 Harb, M. E., Treat, J. W., Ward, B. C., ibid pp 795-806.
In addition, the resin used typically had a particle size such that it would pass through a 35 mesh screen. Resin with smaller particle size (passing through a 100 mesh screen) was found to be extremely difficult to mold, with the articles molded from 100 mesh resin exhibiting severe cracking, so as to render them useless for any testing or utility.
Also, molded articles made via matched metal die compression molding exhibited significant blistering and dimensional distortion when exposed to temperatures of 900.degree. F. for as little as 5 minutes. This phenomenon greatly limits the utility of these articles in high temperature resistant applications.